The moment of follicular rupture was observed during 6 cycles in 5 macaques which were over 6 yr old, anesthetized and weighed 6.8-11 kg. In all cases, the observations were conducted with as little manipulation of the ovary as possible to avoid the possibility of induced rupture of the follicle. Ovulation occurred in the afternoon in all 6 cycles and 2 types of rupture were distinguished. Type I was observed in 3 cycles in 2 monkeys and was characterized by a rapid exudation of surface follicular fluid. In the 3 examples of Type II rupture, the follicular fluid was exuded gradually through one of the reddish diffuse areas which appeared on the surface of the preovulatory follicle. The smooth and tense surface of the preovulatory follicle in M. fuscata suggests that the intrafollicular fluid may exert pressure on the follicular membrane. The reddish diffuse area typical of the Type II ruptures apparently differs from the stigma, a circumscribed area edged by a definite ring of vessels, observed in the rat and has never been reported for other primate species. The diffuse area is presumably closely related to ovulation and may be a weakened section of the follicular wall, although the cause of its appearance cannot yet be explained.